1. Field of the Invention
The present invention relates to an improvement of an antenna system for a rear window glass of an automobile comprising a defogging electric heating element and an antenna.
2. Description of the Prior Art
A whip antenna has been widely used as an antenna for a radio receiver in an automobile. Recently, instead of the whip antenna, a glass antenna system such as AM broadcast antenna strip and FM broadcast antenna strip formed on or in a window glass of an automobile has been practically used.
The glass antenna system has not troubles of the whip antenna such as breaking of antenna by contacting with an obstacle or by bending in parking and deterioration of characteristic caused by dirt staining. However, when the antenna strip is formed on certain window glass, for example, at the upper edge of the window glass or at an upper space above a defogging electric heating element on the window glass, the space for the antenna strip is narrow to reduce the gain of the antenna disadvantageously.
The glass antenna system of an automobile has high directivity characteristics for receiving broadcast waves especially FM broadcast wave in comparison with the whip antenna whereby FM broadcast wave is not satisfactorily received by the glass antenna system in certain directions of travel of the automobile.
For example, when symmetrical antenna strips (3) are disposed at the upper space of the defogging window glass (1) having a plurality of printed heating strips (2) (electric resistance strips) and a feeding point (4) connecting to the lead wire junction (5) are disposed at the center of the rear window glass of the automobile as shown in FIG. 1, the characteristics such as the directivity characteristic curve A shown in FIG. 2 is given to FM broadcast wave such as 80 MHz. As it is clear from FIG. 2, the directivity characteristic curve of the glass antenna system shows a FIG. 8 characteristic having dip point in the case of coinciding with the X direction of the front-to-rear transmitting antenna and the direction of the automobile Y. The gain at the dip point is remarkably low such as 9.7 dB in comparison with the gain at the maximum point such as 45.4 dB. The FIG. 8 characteristic is usually found in the conventional glass antennas and is not only for the above-mentioned design of the glass antenna system. When the glass antenna system is used as a front window glass or a rear window glass, the FIG. 8 characteristic is significantly affected.
The inventors have studied on the directivity characteristic of the glass antenna system to FM broadcast wave. As the result, the inventors have found that the FIG. 8 characteristic of the glass antenna system formed on a front or rear window glass of an automobile to FM broadcast wave is highly affected by the configuration or design of the antenna, and the shape and a size of a body of an automobile since the secondary radiation of FM broadcast wave is caused by the body of the automobile.
When a high frequency amplifying circuit for amplifying high frequency current induced in the glass antenna system is used so as to increase sensitivity of the glass antenna system, the effect of needless waves causes noise. For example, strong TV waves are given in the frequency region just above FM radio broadcast wave band region whereby the strong TV waves adversely affect AM and FM broadcast receiving to cause the noise.